POPACS is a multi-discipline nanosatellite mission and a collaboration between several US universities and entities including MSU (Morehead State University), Gil Moore (POPACS Project Director), the University of Arkansas, PSC (Planetary Systems Corporation), Silver Spring, MD, MSU (Montana State University), Drexel University (Philadelphia, PA), USU (Utah State University), et al. The prime objective of the mission is to measure the changes in density of the auroral zone upper atmosphere, in response to various solar stimuli, such as flares and CMEs (Coronal Mass Ejections).

The mission consists of deploying three aluminum spheres of10 cm diameter into an elliptical polar orbit and tracking them as they make repeated perigee passes in the upper atmosphere, with special emphasis on perigee passes within both the northern and southern auroral zones. The spheres have identical external dimensions and surface compositions, and thus identical drag coefficients, but they have different masses (1, 1.5, 2 kg) to vary their ballistic coefficients, which will cause them to spread out in right ascension along the original orbit. 1)2)

The Aluminum spheres are coated with space-rated flat white paint (AZ-93 white paint, Figure 1) so they can be both radar tracked by the U.S. Space Command and optically tracked by a world-wide network of university students and amateur observers with "Go To" telescopes. The observers will track the spheres, exchange their observations with each other, and calculate the density of the atmosphere above 325 km at the location of the perigee passes.

Background: Originally, the POPACS design consisted of a single 10 kg sphere with a volume of 524 cm3. The original sphere was deployed by what Gil Moore called a heavy duty P-POD. As the Cal-Poly standard became accepted as "The CubeSat standard" by those in industry and the DoD, it was suggested that Gil Moore redesign POPACS to fit to that standard. Hence, the POPACS experiment was restructured making it a 3U CubeSat with 3 spheres. 3)

The POPACS project is an upgraded continuation of project Starshine, in which mirrored ½ m and 1 m spheres were deployed from Space Shuttles into 51.6º, 325 km circular orbits and from an Athena I ELV into a 67º, 450 km circular orbit during Solar Maximum 23 in 1999-2001. These mirrored satellites were radar tracked by the U.S. Space Command and by a worldwide eyeball network of student observers. Data from those missions were used by scientists at the Naval Research Laboratory to refine their NRL MSIS (Mass Spectrometer and Incoherent Scatter) radar codes for determining variations in atmospheric density in response to solar storms.

Spacecraft:

The spacecraft is a 3U CubeSat enclosure as shown in Figure 2 containing the three spheres along with the spacers as shown in Figure 7. The enclosure must cradle the spheres firmly enough to withstand launch conditions but gently enough to not damage the sphere surface. The spacers must be able to deploy three spherical structures upon command. The total mass of the 3U CubeSat is ≤ 6 kg.

The outer shell of each hollow POPACS sphere has been machined by C & G Machine of Colorado Springs, CO to a thickness of 1 cm and a mass of 680 gram from Aluminum 6061-T6, which will readily melt upon atmospheric reentry at an altitude of ~80 km.The spheres have also been ballasted to their design masses of 1.0 kg, 1.5 kg and 2.0 kg with tiny particles of Bismuth shot, which will also melt at reentry, and sand, which will be harmlessly dispersed at reentry.

Figure 3: Section of a POPACS sphere design (left) and the actual hemispheres of the final spheres (right), image credit: POPACS consortium

CSD (Canisterized Satellite Dispenser):

One of the mission enablers is the 3U CSD designed and developed by PSC (Planetary Systems Corporation). While having similar internal dimensions to the P-POD (CubeSat launcher) design, the 3U CSD has key features that provide higher payload mass capability (6 kg), tabbed preload system to guarantee a stiff and modelable load path to the CubeSats, a higher ejection velocity, lower overall volume, mounting features to allow fastening of the CSD at any of the six faces, a 15 pin in-flight disconnect allowing battery charging and communication from the outside of the CSD into the payload while in the launch pad or on orbit, and a reusability that allows a separation test to be conducted at will without any consumables hundreds of times to guarantee reliability. The CSD will be qualified to levels exceeding the MIL-STD-1540 standard for thermal vacuum, vibration and shock. 4)

The POPACS payload is rigidly fastened to the CSD via the tabs (Figure 5) which are preloaded when the door is closed. This guarantees a stiff and modelable load path and avoids potentially detrimental jiggling. Preloading the tabs (which is what bolted joints do to flanges of adjoining structures) will be of great use in vibration and shock environments where model ability could preempt test failure by accurately predicting problematic modal frequencies and responses.

The CSD incorporates the initiation connector (used by the launch vehicle to open the door) and in-flight disconnect (to communicate from the Launch Vehicle to the payload). The door is restrained and initiated with an automatically resettable latch driven by a reusable initiator (Figure 6). By closing the door, the payload is preloaded to the CSD and ready for flight (Ref. 1).

Launch: POPACS was launched as a secondary payload on Sept. 29, 2013. The primary payload on this flight was CASSIOPE of Canada. The launch vehicle was the Falcon 9 v1.1 of SpaceX (maiden flight of Falcon 9 v1.1 configuration). The launch site was VAFB, CA. 5)

Immadiately after dispensing, the spheres are freed from the spacers - and the science mission begins.

POPACS experiment:

The apogee of each sphere's orbit will shrink a slight amount each time the sphere passes through perigee of its orbit, due to atmospheric drag, with the lightest sphere experiencing the greatest rate of orbit decay. This orbit decay can be interpreted as a direct measure of atmospheric density above 325 km. The drag will also cause the three spheres to spread out in right ascension because of their different ballistic coefficients. At the same time, apsidal regression will slowly cause the perigees of the spheres to separate from each other in azimuth. The result of these effects will be that the sphere's perigee passes will occur at approximately 100 minute intervals at three different times and three different geographic locations in the earth's upper atmosphere, thus increasing the breadth and frequency of the density measurements.

Figure 8: Computer simulation of the orbital decay of the POPACS orbits three years after launch (image credit: Analytical Graphics, Inc.)

Approximately ten years after launch, depending on the way the sun behaves during solar cycles 24 and 25, the orbit of the 1 kg sphere will become circular at an altitude of 325 km and will shortly thereafter deorbit and be consumed, while the 1.5 kg sphere will deorbit in 12.5 years, and the 2 kg sphere will deorbit in 15 years.

Tracking: The U.S. Space Command will radar track the metallic spheres throughout their lifetimes and periodically publish their TLEs (Two-Line orbital Element sets). Student groups in participating universities around the world will use these element sets to optically track the spheres with "Go To" telescopes and calculate their own orbits of the spheres, measure the way in which the orbits decay, and thereby determine the density of Earth's atmosphere at the location of each sphere's perigee passage. The students will pay particular attention to the way in which the density of Earth's upper atmosphere in its northern and southern auroral regions responds to solar flares and CMEs (Coronal Mass Ejections) and the way that those effects propagate equator-ward. Software for computing these effects will be provided to the students by the Center for Space Standards and Innovation of Analytical Graphics, Inc. in Colorado Springs.

The E.O. Hulburt Center for Space Research at the NRL (Naval Research Laboratory) in Washington DC will make their own computations of atmospheric density from the Space Command's tracking data, and will provide their results to the students for comparison purposes. Prizes will be awarded by the POPACS project to the student groups whose results most closely match those of NRL (Ref. 1).

Mission status:

• August 2016: Satellite drag variability caused by the dynamics of the upper atmosphere is a major cause of orbit specification and prediction errors in Low Earth Orbit. The problem is particularly severe during geomagnetic storms. These storms can severely degrade the accuracy of conjunction analysis between debris and spacecraft with LEO perigees and all other resident space objects. POPACS was launched as secondary payload on the September 29, 2013 on a Falcon-9 vehicle. The objective is to provide a stable and well defined atmospheric calibration, leading to improved specification of atmospheric densities and satellite drag. 6)

- POPACS consists of three spheres deployed successfully from a Planetary Systems Corporation CSD (Canisterized Satellite Dispenser) 3U CubeSat ejection system. Specially designed spacers separated the spheres during launch and deployment. After summarizing the POPACS mission parameters and design, early mission validation results are reviewed including data comparisons with the Drag and Atmospheric Neutral Density Explorer as well as comparisons with various atmospheric models.

- The research team also look at the ability of the POPACS data for alerts to large changes in atmospheric densities and satellite drag during geomagnetic storms. Finally, it is shown that well-designed reference satellites, such as POPACS, can be used to calibrate the state of the atmosphere. This calibrated data can be used to improve global atmospheric modeling and orbital predictions for both space debris and active satellites.

Figure 9: Numerical prediction of the orbital decay of the POPACS orbits three years after launch (image credit: Wes Bradley)

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).